The present invention generally relates to an external combustion engine which is a kind of a heat engine, and, more particularly, to an external combustion engine of the Stirling engine type.
The Stirling engine referred to above is generally comprised of a hermetically sealed vessel, a working fluid such as helium or hydrogen sealed in the vessel, a heater for heating the working fluid, a cooler for cooling the working fluid, a regenerator which stores heat while the working fluid reciprocates between the heater and the cooler, and a mechanical means such as a piston or the like which is driven by the working fluid. The working fluid is continuously heated and cooled while making a reciprocal movement between the heater and the cooler and, as a result, the working fluid is repeatedly compressed and expanded. Because of the aforementioned structure in the Stirling engine, the working fluid acts on an external load while making changes intimate to the Stirling cycle.
The construction of the prior art Stirling engine is as shown in FIG. 4.
More specifically, in the prior art Stirling engine, a working fluid such as helium or hydrogen is hermetically filled in a sealed vessel formed by walls 1 and 2. The Stirling engine includes a pipe 3 to be heated for heating the working fluid, a cooler passage 4 in which the working fluid is cooled, and a regenerator matrix 5. A displacer 6 is movable up-and-down so as to reciprocate within the wall 1, while maintaining a small clearance from the inner surface of the wall 1. Moreover, a piston 7 is arranged to be slidable over the inner surface of the wall 2 when undergoing up-and-down reciprocation. The piston 7 acts on a load 8 of a linear alternator, a compressor or a pump, etc., which is secured at one end to the inner surface of the wall 2, and at the other end to the piston 7. Springs 9 and 10 support the wall 2 on a base 11. Compression spaces 17 and 19 communicate with each other through passageways 20 and 21.
Meanwhile, the wall 1 is fixed to the wall 2 by a plurality of sets of bolts 12 and nuts 13. An insulation member 14 is sandwiched between the walls 1 and 2.
The outer surface of the pipe 3 is heated by a heat source such as combustion gas of fossil fuels or solar energy.
The working fluid in the cooler passage 4 is cooled by a coolant which enters from a pipe 15 to run along the outer wall of the wall 2 to a pipe 16.
The operation of the prior art Stirling engine having the above-described construction will now be explained.
When the displacer 6 is lowered, the cubic volume of the compression space 17 is reduced, while that of an expansion space 18 is increased. Therefore, the pressure in the compression spaces 17 and 19 becomes higher than the pressure in the expansion space 18. As a result, because of this difference in pressure, the working fluid at low temperatures in the compression spaces 17 and 19 and the cooler passage 4 is, through the regenerator matrix 5 and the pipe 3, sent towards the expansion space 18. During this time, the working fluid is heated by the regenerator matrix 5 and the pipe 3. On the contrary, the regenerator matrix 5 is cooled. Thus, since the working fluid at low temperatures is heated, the pressure in the space filled with the working fluid over the piston 7 (hereinafter referred to as a working space) is raised, thereby drawing the piston 7. At this time, the piston 7 down acts on the load 8. On the other hand, when the displacer 6 continues to be lowered, the pressure in a gas spring 22 is gradually increased. And finally, the displacer 6 stops, and then, conversely, begins to ascend. When the displacer 6 is raised, the cubic volume of the compression space 17 is increased, while that of the expansion space 18 is reduced. Due to this change, the pressure in the expansion space 18 becomes higher than that in the compression spaces 17 and 19. The difference in pressure brings the working fluid at high temperatures in the expansion space 18 and the pipe 3 towards the compression space 17 through the regenerator matrix 5 and the cooler passage 4. At this time while the working fluid flows from the expansion space 18 to the compression space 17, it is cooled by the regenerator matrix 5 and the inner wall of the cooler passage 4. The regenerator matrix 5 is conversely heated. As described hereinabove, the working fluid at high temperatures is cooled, and accordingly the pressure in the working space is lowered, thereby raising the piston 7. At this time, the piston 7 drives the load 8.
In the meantime, if the displacer 6 continues to ascend, the pressure of the gas spring 22 is gradually reduced, until the displacer 6 stops and then begins to lower. In the process of one cycle as above, the working fluid changes a part of the heat added through the pipe 3 to drive the load 8, while leaving a part of the heat to the inner wall of the cooler passage 4.
The heat entering from the heat source into the pipe 3 is largely included in the working fluid. However, it partially flows through the pipe 3 to the wall 1, and further through the bolt 12 and the nut 13 to the wall 2 to be transmitted to the coolant.
In the manner as described above, a part of the heat entering from the heat source to the pipe 3 is transmitted not to the working fluid, but to the coolant. The reason for this is that the temperature of the heat source is higher than the temperature of the coolant. Although the insulation member 14 prevents some heat from being transmitted from the wall 1 to the wall 2, some actual heat is transmitted from the wall 1 to the wall 2 through the bolt 12 and the nut 13. Thus, such heat is actually present that is not transmitted to the working fluid, but is transmitted to the coolant. Therefore, the thermal efficiency of the prior art Stirling engine is disadvantageously low.